VSEPR minimum-energy structures

Finally we turn our attention to the suboxide. Experiments show that the atoms are connected in the sequence OCCCO. The Lewis structure of is presumably 0 C C C 0 . The observed C-C and C-O bond distances are indeed consistent with a double bond description 0=C=C=C=0. The VSEPR model then leads to the prediction that the molecule is linear. This is not strictly true the molecule is bent at the central atom with ZCCC = 156° The potential energy maximum at ZCCC = 180° (see Fig. 9.1) is, however, less than 0.25 kJ mol above the minimum, and the molecule is therefore often described as pseudo linear [1]. Why the small energy hump at ZCCC = 180° is there, nobody seems to know. 

Then, since the electronic total energy is a negative measure (of minimum) of the stability of a structure, the relation (2.198) for electronic density will be controlled, eventually, by the topology (space distribution) of Laplacian (A) of electronic density, with minus sign, i.e.,-Ap r). On the other side, the fact that the VSEPR paradigm takes in consideration the electron pairs and not the individual electrons should be quantified, for which reason a particular measure for the inter-separation of the electron pairs will be introduced. 

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